ui: switch from join to left join when querying thread_tracks for flows
ui: switch from join to left join when querying thread_tracks for flows

Bug: 228587276
Change-Id: I7965270cfbf9959205c447551ba5c30a48297107
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto